This invention is directed to high surface area monolithic structures composed of sintered ceramic oxide materials which have high porosity. The structures are useful as filters for fluids and as catalytic substrates in that they provide high surface area for particular filtration or for deposition of catalytic material. The invention is more particularly directed to structures in which the ceramic material is primarily alumina, titania, or zirconia which has been modified, prior to firing or sintering, by admixture with a phosphate material that generates P.sub.2 O.sub.5 upon heating. The structures are particularly useful as catalyst supports in the conversion of automotive exhausts and in reduction of NOx emissions from industrial sources, and as fluid filters, such as those used in diesel engines.
Conventional monolithic ceramic catalyst supports consist of an underlying ceramic support material with a coating of high surface area material upon which the catalyst itself is actually deposited. In particular, the ceramic support is prepared by sintering a mold of clay or other ceramic oxide (alumina, titania, cordierite, etc.) at a temperature sufficiently high to densify and strengthen the material. Temperatures high enough to result in effective sintering, however, also cause pore shrinkage and other microstructural changes that result in the sintered material's having a very low surface area. Consequently, the sintered ceramic must be coated with another material having a higher surface area, often a ceramic material itself that has not been sintered or pre-reacted, on which to actually deposit the catalyst. This procedure of applying a high surface area "wash-coat" on the low surface area ceramic wall is disclosed, for example, in U.S. Pat. Nos. 2,742,437 and 3,824,196.
In addition to the exposure to high temperature during sintering, however, catalyst support structures can also be exposed to elevated temperatures in service. The surface area of a wash-coat can be substantially degraded, and the surface area of the underlying ceramic may also further be reduced in some instances, because of the high service temperatures, such as those of automotive exhaust gases, to which they are exposed. It is therefore desirable to use ceramic materials that are, or can be modified to be, resistant to loss of surface area when exposed to elevated temperatures either during firing or service. One such material is a mixture of 50-93% by weight alumina and 7-50% by weight silica as disclosed in U.S. Pat. No. 4,631,269 (Lachman et al, issued Dec. 23, 1986).
It is an object of the present invention to provide an improved monolithic structure that can be sintered to provide structural strength and integrity without loss of appreciable surface area. It is a further object of the invention to provide a structure that resists thermal degradation of its porosity and available surface area despite exposure to elevated temperatures in catalytic conversion processes.